This invention relates generally to devices for controlling fluid flow. More specifically, the invention is directed to valves which are capable of detecting and stopping the flow of liquid in a polyphase fluid stream.
A wide variety of conduits carry fluids from one location to another. For example, conventional pipelines carry air, water, oil, or other liquids and gaseous media. Networks of pipelines in the United States and other countries throughout the world handle the cross-country transportation of crude oil, refined petroleum products, and natural gas. Oil, gas, and steam pipelines often deliver these fuels directly to power-generation equipment, such as steam turbines and gas turbines.
Turbines and other power generators frequently rely on the gaseous fuels to generate mechanical energy and/or electricity. Over the course of many decades, the size and sophistication of this type of equipment has increased dramatically. Its smooth operation and longevity depend in part on the composition and quality of fuel being supplied by pipes or other conduits. (The fuel is typically hydrocarbon-based, and made up of a host of components, e.g., methane, butane, octane, and various mixtures which may contain these or other components).
The inner mechanism of a turbine usually involves rotating members (e.g., rotors) and stationary members, along with associated components. The turbines, like other power-generating equipment, can be specifically designed to run on a gas fuel with a certain heat capacity, e.g., in BTU per cubic foot. Equipment manufacturers often specify that the fuel has to have a specific temperature at the point of entry—high enough to prevent or minimize any condensation, e.g., 50° F. “superheat” being a common requirement.
The fuel mixture is sometimes in the form of a polyphase fluid moving through the pipe. For example, two-phase flow (usually gas and liquid) is often common when the fuel source is somewhat unprocessed. Two-phase flow is often a long-standing problem when the fuel is obtained directly from gas fields, for instance. As another example, the fuel may become contaminated by equipment lubricants, fuel treatment agents, or other sources. The design parameters for turbines may allow for a certain, minimal amount of liquid and other foreign components in the fuel composition. For example, a gas flow stream having minor amounts of liquid droplets suspended in a somewhat evenly-distributed manner within the stream may not adversely affect a typical gas turbine.
However, greater amounts of liquid or condensed material in the fuel stream can adversely affect turbines and similar types of equipment. For example, the gas stream may include “slugs” of liquid material. The slugs are usually a natural result of the condensation of the fuel gas itself into liquid fuel, and they are typically in the form of condensed, detached masses. Hydrocarbon liquid slugs have BTU values much greater than a pure gas composition. If the slugs are carried into the turbine, they will cause firing temperatures within the precisely-calibrated machinery to increase dramatically and uncontrollably. At a minimum, the turbine may be mechanically damaged, necessitating considerable repair cost and down-time. In some instances, the impact of the liquid slugs can actually result in the melting of the internal turbine mechanism, thereby destroying the equipment. Moreover, the passage of non-combustible slugs (e.g., aqueous slugs) into the turbine may result in thermal shock, which can also lead to extensive damage.
Certainly, steps have been taken in the past to prevent damage to power equipment from liquid slugs. The machines are usually temperature-protected, e.g., with sensors which detect abnormal temperature increases. The detection mechanism is linked to other damage-prevention systems, which can shut off the fuel stream, and/or shut down the equipment.
Moreover, knock-out drums are typically incorporated into gas fuel lines, upstream of the power generator equipment. These containment systems are designed to capture liquid and liquid slugs in the gas stream. They then contain all of the undesirable liquid components within holding tanks or “drums”. Sensors attached to the drums provide an indication of liquid levels, so that steps can be taken to empty the drums before they reach capacity. The system is usually automatic, with an alarm-notice to operators.
However, shut-off mechanisms activated by temperature sensors are incapable of shutting off an improper fuel stream in time to prevent damage to the power equipment. Moreover, knock-out drums can easily become over-filled by a sudden rush of slug flow. The monitoring systems and alarm-mechanisms on the drums may not be sufficient to shut off fluid flow in time to prevent damage to the equipment. Additionally, the knock-out drums are difficult to maintain to a standard of 100% reliability.
Different types of valves in conduits have been used in the past to address the problem of liquid slug flow. For example, in U.S. Pat. No. 2,410,984 (Lawless), a valve is incorporated into a steam line. The valve prevents the surge of water and other liquids which would otherwise damage machinery operated with the steam. The valve includes a hollow sleeve with many perforations, which allow the passage of steam. The cylindrical sleeve is slidably mounted on the valve body, and is closed at one end with a disc. A spring mechanism is biased to keep the sleeve in a position which allows uninterrupted flow through the steam line. If a surge of water moves through the line, the extra force overcomes the spring tension, causing the disc to move against a valve seat. In this manner, the valve is effectively closed, thereby cutting off the flow of both steam and water through the line.
While the invention of Lawless may be suitable for preventing undesirable liquid flow in some situations, it also has some serious drawbacks. For example, the valve described therein requires an internal sleeve or piston, along with the mechanism needed to move the sleeve. These elements would be routinely exposed to any type of fluid moving through a pipe, along with solvents, dirt, or other contaminants. Such exposure would probably require a substantial maintenance effort to keep the components of the valve in operating condition, e.g., to prevent the fouling of the sliding region of the sleeve.
Moreover, it appears that the fluid in the Lawless patent must move through the perforated region of the sleeve. While these apertures may be suitable for a steam line, they are probably impractical when the fluid is natural gas or other hydrocarbons. The perforations would drastically reduce flow rates required for the efficient delivery of gaseous fuel through present-day pipelines.
Another type of valve is described in U.S. Pat. No. 4,301,833, issued to Donald, III. The Donald patent describes a valve which shuts off gas or liquid flow through a conduit when the flow rate increases beyond a pre-set limit. The valve includes a flat plate member, which can be pivoted eccentrically. In this manner, the plate can swing between a normal flow-rate position and a pipe-closure position. When flow rates through the pipe exceed a threshold amount, the forces on the plate apparently cause it to swing and close with a rapid snap action. An attached bias spring can be used to set the threshold level at which the plate will swing to the closed position.
While the invention of Donald may be effective for shutting off liquid and gas flow under some conditions, there are considerable drawbacks as well. The valve in this patent is designed to sense only changes in fluid velocity, and not fluid force or density. Therefore, the Donald valve would appear to operate under a very narrow set of conditions. For example, the bias spring could be set to release the plate at one particular flow speed. If a full-flow threshold were selected, then a “half-flow” condition would apparently not shut the pipe, even though those conditions might be allowing undesirable materials (e.g., liquid slugs) to move through the pipe. Conversely, if a half-flow threshold were selected, the valve might shut off flow prematurely, even though the fluid contents were not necessarily undesirable.
U.S. Pat. No. 3,380,474, issued to Mills, describes a “flap” valve device, for restricting fluid flow when the flow exceeds a given value. The interior of the valve is shaped to include a valve seat, toward which the fluid flows. A flapper within the valve is normally urged in an open position by means of a spring. However when fluid flow exceeds a pre-set limit, the restraining force of the spring is overcome, and the flapper is urged downward to engage the valve seat and shut off fluid flow.
While the invention of Mills may be useful in some cases, there are serious drawbacks if the device were employed in some of the situations described above. For example, the movement of the spring-controlled flapper depends only on the velocity of fluid flowing through the valve. Thus, the mechanism in that patent cannot readily discriminate between gas flow and the flow of liquid slugs, and could prematurely shut off fluid flow, resulting in the problems described herein.
Still another shut-off valve is described in U.S. Pat. No. 6,581,629, issued to Eielsen. The valve includes a housing, through which a flow passage extends. A valve body is positioned within the housing, and includes an airfoil-shaped end portion which is capable of extending into the flow passage. Under normal fluid-flow conditions, the valve body and its end portion rest on the bottom surface of the flow passage, keeping the passage generally clear. Under abnormal flow conditions, the fluid provides a lifting force which appears to cause the valve body and its end portion to pivot upward, thereby blocking fluid flow.
The shut-off valve of Eielsen could be useful for controlling fluid flow in some pipelines and other types of flow passages. However, the valve may exhibit considerable disadvantages under some flow conditions. For example, the force required to move the valve body to the blocking position results from the differential in pressure at the airfoil surface, as fluid is accelerated over the surface. (Density changes may occur as a result of undesirable liquid slugs which may be flowing within the gaseous stream). However, it appears that the lifting force for the Eielsen valve may greatly decrease as the distance from the airfoil surface increases. Thus, density changes (from liquid slugs) which do not occur immediately near the airfoil surface may not trigger the valve to shut off the flow.
Furthermore, as described in Eielsen, the valve mechanism seems to depend on the use of various communication channels, hydraulic channels, and other structural members situated beneath the valve body. (The pivoting movement of the valve body is controlled by the movement of hydraulic fluid in chambers below the body). These types of features would appear to require various shaft bearings and seals for the channels and chambers. The required bearings and seals can be the source of serious maintenance problems during the use of the valve system. This is especially problematic when the associated pipeline is carrying fluids which can corrode or otherwise weaken the seals and bearings.
In view of the discussion above, it appears that new types of shut-off valves would be welcome in the industry. The valves should be capable of readily detecting the presence of a liquid flowing in a polyphase stream, e.g., a gas pipeline susceptible to liquid slug flow. The valves should also be capable of immediately stopping the flow of all fluid through the pipeline when the liquid material is detected. Moreover, in most instances, the valve mechanism which senses liquid flow and shuts off all fluid flow should be relatively simple in construction. For example, the mechanism should be free of (or have only a minimum of) additional valve chambers, shafts, seals, or other features which could increase maintenance concerns, or which could adversely affect long-term pipeline- or valve-integrity.